The present invention relates generally to exposure apparatuses, and more particularly to illumination and exposure apparatuses used to expose an object to be exposed, such as a single crystal substrate for a semiconductor wafer, and a glass substrate for a liquid crystal display (LCD), a device fabricating method using the exposed object, and a device fabricated from the exposed object. The present invention is suitably applicable, for example, to step-and-scan, scanning, or step-and-repeat exposure apparatuses which expose a single crystal substrate for a semiconductor wafer in a photolithography process.
The xe2x80x9cstep-and-scanxe2x80x9d manner, as used herein, is one mode of exposure method which exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask or reticle (hereinafter, these terms are used interchangeably in this application), and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The xe2x80x9cscanningxe2x80x9d manner is another mode of exposure method which uses a projection optical system to project part of a mask pattern onto a wafer, and exposes the entire mask pattern to the wafer by synchronously scanning the mask and wafer relative to the projection optical system. The xe2x80x9cstep-and-repeatxe2x80x9d manner is still another mode of exposure method which moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer.
Along with the recent demand on smaller and thinner profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern tries to achieve a line and space (LandS) of 130 nm on a mass production line, and predictably it will be increasingly smaller in the future. LandS denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution. The exposure has three important factorsxe2x80x94resolution, overlay accuracy, and throughput. The resolution is the minimum size for a precise pattern transfer. The overlay accuracy is a precision with which to overlay multiple patterns over an object to be exposed. The throughput is the number of sheets exposed per unit of time.
Basically, there are two exposure methodsxe2x80x94a full-size transfer method and a projection method. The full-size transfer includes a contact method that brings a mask into close contact with an object to be exposed, and a proximity method that slightly spaces them from each other. Although the contact method may provide the high resolution, dusts and silicon fragments enters under the mask in a compressed state, causing the mask to be damaged as well as the exposed object to be flawed and defective. The proximity method ameliorates such problems, but still possibly damages the mask if a distance between the mask and the object to be exposed becomes shorter than the maximum size of a dust particle.
Accordingly, a projection method has been suggested which farther spaces the mask from the object to be exposed. Among projection modes, a scanning projection exposure apparatus has been the recent trend for the improved resolution and expanded exposure area, which exposes the entire mask pattern onto the wafer by projecting part of a mask, onto the wafer and synchronizingly scanning the mask and wafer with each other, continuously or intermittently.
In general, a projection exposure apparatus includes an illumination optical system that uses a beam or ray emitted from a light source to illuminate a mask, and a projection optical system that is located between the mask and the object to be exposed. In order to form a uniform illumination area, the illumination optical system introduces the beam from the light source to a light integrator, such as a fly-eye lens, which includes a plurality of rod lenses, and uses a condenser lens to Kohler-illuminate the mask surface with a plane of exit side of the light integrator as a secondary light source plane.
The following equation gives the resolution R of the projection exposure apparatus using a light-source wavelength xcex and the numerical aperture of apertures (NA) of the projection optical system:                     R        =                              k            1                    xc3x97                      λ            NA                                              (        1        )            
Therefore, the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes.
In the meantime, a focusing range that maintains desired image-forming performance is called a depth of focus (DOF), and the DOF is given in the following equation:                     DOF        =                              k            2                    xc3x97                      λ                          NA              2                                                          (        2        )            
Therefore, the shorter the wavelength becomes and the higher the NA increases, the smaller the DOF becomes. The small DOF would make difficult the focus adjustment, as well as require the higher flatness for a substrate and the more precise focusing accuracy, and thus the large DOF is basically desirable.
It can be understood from these equations 1 and 2 that the shortened wavelength is more effective than the increased NA. Therefore, in recent years, a light source is in transition from the conventional ultrahigh pressure mercury lamp to short-wavelength KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). As the number of optical members that transmit light increases, the shortened wavelength of exposure light lowers the transmittance (and thus the luminous intensity at an image plane and throughput due to decreased power), and therefore, recent projection exposure apparatuses attempt to use a smaller number of optical members for high transmittance. In addition, when a comparatively small ultrahigh pressure mercury lamp is used as a light source, a projection exposure apparatus usually can mount the light source in the apparatus body. On the contrary, when a comparatively bulk excimer laser is used as a light source, the projection exposure apparatus cannot mount the light source in the apparatus body, and the light source has to be separated from the apparatus body (for example, such that the apparatus body is located on a second floor and the light source unit is located on the first floor).
Such a conventional configuration that separates the apparatus body from the light source unit has a problem in that optical axes shift due to different installation environments between the apparatus and the light source unit, lowering the resolution for the exposure. For example, when these apparatus and unit are located on different floors, the different installation environments (such as vibration frequency and phase on each floor) would easily result in discordant optical axes and the fluctuant and uneven luminous intensity at the time of exposure, lowering the resolution. This is because the beam incident upon the illumination optical system that has a shifted optical axis causes the vignetting as well as fluctuant luminous intensity, and a beam that asymmetrically enters the illumination optical system causes the uneven luminous intensity. The fluctuant and uneven luminous intensity hamper control of the exposure amount to the desired amount at the time of exposure, thereby deteriorating the resolution. In particular, in light of the recent demand on the minute pattern, it is expected that even a slight degradation in resolution cannot be neglected in the future.
In order to solve these problems, Japanese Laid-Open Patent Applications Nos. 11-145033 and 2000-77315 propose means for correcting an optical axis in real time. Both references propose to correct an angular shift in the optical axis by pivoting a mirror, and a positional shift in the optical axis by tilting a parallel plate of a specified refractive index. Although the rotation of the mirror attains sufficiently responsive correction, the plane parallel plate undesirably lowers the transmittance and thereby throughput. On the other hand, it is conceivable to correct a positional shift in the optical axis by moving the mirror in parallel, but it is difficult to quickly move the mirror in parallel while keeping the mirror angle, disadvantageously resulting in poorly responsive correction.
Accordingly, it is an exemplified general object of the present invention to provide a novel and useful correction apparatus, exposure apparatus, device fabricating method, and device, in which the above disadvantages are eliminated.
More specifically, an exemplified object of the present invention is to provide a correction apparatus which corrects shifts in optical axes with good responsiveness but without deteriorating the transmittance from a light source (i.e., while keeping the desired throughput), an exposure apparatus and device fabricating method which use the correction apparatus to enhance the performance. Of course, the present invention cover those devices, such as high quality semiconductors, LCDs, CCDs, and thin film magnetic heads, which are manufactured by the device fabricating method.
In order to achieve the above object, a correction apparatus according to one aspect of the present invention, which corrects a shift between optical axes in two separate optical units adapted such that a beam emitted from one unit enters the other includes a converter for converting a positional shift between the optical axes into an angular shift, and an angular corrector for correcting the angular shift. According to such a correction apparatus, the converter converts the positional shift into the angular shift, and eliminates the need to correct the positional shift between the optical axes, for example, by moving a reflective point in parallel.
The present invention is applicable to two optical units that are not separated, and in this case a shift between optical may be broadly extended to a shift between optical paths or beam positions. For example, the present invention is applicable to two optical units located in the same room, one of which is a laser source, so as to correct a shift between optical paths or beam positions when the laser source vibrates.
A correction apparatus according to another aspect of the present invention, which corrects a shift between optical axes in two separate optical units adapted such that a beam emitted from one unit enters the other includes a first angular corrector for correcting an angular shift between the optical axes, a converter for converting a positional shift between the optical axes into an angular shift, and a second angular corrector for correcting the converted angular shift. This correction apparatus also uses the converter to convert the positional shift into the angular shift, eliminating the need to correct the positional shift between the optical axes by using the refraction or by moving a reflective point in parallel.
At least one of the first and second angular correctors includes a reflective mirror, and a tilting mechanism for tilting the reflective mirror with respect to one of the optical axes. Such a structure has a good responsiveness and few losses at the time of correction. The reflective mirror may be replaced with an optical member using total reflection utilizing a difference between refractive indexes. This is because it is difficult to design of a reflective film having high reflectance for use in a short wavelength range since only limited types of materials are available to form such a reflective film, and the total reflection utilizing a difference between refractive indexes of the optical member and air possibly provides higher reflectance.
A correction apparatus may further include a first detector for detecting the angular shift between the optical axes, a second detector for detecting the converted angular shift corresponding to the positional shift between the optical axes, and a controller, connected to the first and second detectors, which controls the first and second angular correctors based on detection results from the first and second detectors. According to this correction apparatus, the controller may control the optical axis with high precision in accordance with the detection results by detectors. At least one of the first and second angular correctors may include a reflective mirror, and a drive unit for driving the reflective mirror so that the reflective mirror may tilt with respect to one of the optical axes, wherein the controller controls the drive unit. As described above, such a structure is highly responsive with few losses at the time of correction.
According to still another aspect of the present invention, there is provided an exposure apparatus including one of above correction units, and an optical system which projects a pattern formed on a reticle or mask onto an object to be exposed. Such an exposure apparatus also performs operations similar to the above exposure apparatuses.
According to yet another aspect of the present invention, there is provided a device fabricating method comprising the steps of exposing an object to be exposed using the above exposure apparatus, and performing a predetermined process for the exposed object. Claims for a device fabricating method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors such as, thin film magnetic heads, or the like.
Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings.